Devices for large-scale lyophilization of pharmaceutical solutions in medical hollow bodies are known. For economic reasons, the lyophilization process takes place in a temperature range which is as high as possible, but which does not allow thawing or loss of structure of the frozen product matrix. To ensure that the product temperature remains below a defined limit, which depends on the substance to be lyophilized, temperature sensors which are used for characterization and control of the lyophilization cycle are typically introduced into the product. It is disadvantageous that the sensors, on account of their own heat capacity, may influence the temperature variation or distort the drying rate. For this reason, lyophilization processes are frequently designed in such a way that a sufficient safety margin is maintained with regard to the temperature. This means that the process is carried out at an unnecessarily low temperature, resulting in a decreased drying rate. Therefore, in many cases the process lasts much longer than would be necessary if operations could be carried out at the highest possible temperature range. The danger in exceeding a certain temperature limit lies in the fact that the cake formed by the substance to be lyophilized may collapse when the atmosphere of the lyophilizer is selected to be too humid, or the temperature of the frozen material is too warm. Both process parameters, i.e., the humidity of the atmosphere in the lyophilizer and the product temperature, are a function of the saturated vapor pressure of the substance to be lyophilized. Therefore, to avoid collapse of the cake, before starting a large-scale lyophilization it is important to establish process parameters which consistently ensure success of the process. Examples of such process parameters include the product temperature or the temperature of the shelves of the lyophilization unit, the pressure in the lyophilizer, the humidity of the atmosphere within the lyophilizer, the change of these parameters as a function of time over the entire process, and, not least of all, the total duration of the process. It is understood as a matter of course that each of the values to be set is a function of the specific substance to be lyophilized. Thus, if a lyophilization cycle is to be developed for a substance for which there is no previous experience regarding lyophilization, the relevant parameters must be experimentally determined. For this purpose miniaturized lyophilization units may be provided, which may be placed on the specimen stage of a microscope. The course of the lyophilization may be observed, either with the naked eye or using a camera, through the lens of the microscope and a light-transmitting wall of the miniaturized lyophilization unit. In this manner, according to the principle of trial and error, preferably in multiple test series, the process parameters may be determined for which a cake of the lyophilization material collapses, and for which a successful process takes place. The process parameters determined in this way, in particular in the form of limiting parameters to be maintained, preferably taking a safety margin into account, may then be transferred to the large-scale lyophilization; once again, the temperature sensors described above may be used for characterizing and controlling the process. However, as stated, since the temperature sensors adversely affect the process itself, it would be desirable to have an option for process characterization and control which on the one hand does not require direct contact with the lyophilization material, and on the other hand does not adversely affect the process itself. It would also be desirable for the new option for process characterization and control to allow online monitoring in the sense that evaluatable criteria for controlling or regulating the process parameters may be used during the lyophilization process, so that the process parameters may consistently be maintained in an optimal range without having to use limiting parameters determined beforehand in a complicated and laborious manner in separate experiments.